I just measured some 12 g hollow slugs I cast. They ran about .703 - .705 . I forget whose mold it is... I'll try to find it tomorrow.
You might want to get a mold and cast some slugs before you order the barrel.
A thought just crossed my mind... not sure if it is a good one or a bad one, but you maybe could push the tolerances a bit with hollow slugs. The muzzle could get a bit of a chamfer to get the slug started if it isn't perfect. Even if it is perfect, a slight chamfer whould help if the windage is tight due to the square base of the slug. Others may have some thoughts/opinions about that.
Also, there are sabot slugs that are designed to be used in regular plastic shotgun wads, which opens up many new possibilities.